Dakix and Latarche — The Plankton of Lough Neagh. 45 



Oscillatoria- rubesc*:ns has been discovered to be dangerous to the fishery in 

 Zurich See. The filaments form a kind of network with jnud over the fish- 

 eggs on the bottom, and prevent the eggs from reaching the hatching-point, or 

 suffocating the young larvae, As a matter of fact, more than one blue-green 

 alga has been considered as a dangerous element in lakes ; and though it is not 

 known to what extent the development of these forms has increased or 

 remained stationary in Lough jSTeagh, it will be well to remember that they 

 occur in considerable quantity at present. 



The smallest quantity of Oscillatoria is present in the months of September 

 to January ; and, as a matter of fact, the genus seemed to be totally absent 

 from the plankton in January, 1911. 



Oscillatoria Agardhii is recorded by Messrs. West in their seasonal studies 

 of the plankton of the English and Scottish lakes. It is, however, never 

 recorded as more than rare. It is usually absent during many months of the 

 year, and it seems to appear in very different months in different lakes. 

 Against this, it is one of oiu' most common species in Lough Neagh, and 

 present almost the whole year through. 



Under the heading Oscillatoria spp. Kofoid includes several species which 

 may include 0. Agardliii, Gomont, although no special mention is made of 

 this form. Oscillatoria occurred in every month of the year until the 1 st of 

 October. The seasonal curve is most irregular and variable, and Kofoid 

 regards it as a possible adventitious or only temporary planktont. 



Anabaena. 



Anabaena is represented in Lough Xeagh chiefly by the species flos-ctquae 

 (Lyngb.) Breb., though A. Lemmermanni is also recorded by Messrs. West. 



The Cyanophyceae are on the whole characteristic of Central European 

 lakes, and are absent from the Arctic lakes. Analcmui Flos-aqvxie is, how- 

 ever, recorded from north Sweden and Eussian Lapland. It is common in the 

 Central European lakes and the Danish lakes. In Messrs. West's latest paper 

 it is only recorded for Loch Lubnaig in Scotland, and is then noted as 

 occurring only in August. It has, however, been recorded by them for lakes in 

 all parts of the British Islands. There is no doubt that it is on the whole very 

 rare in the Scotch and Welsh lakes. It is, on the contrary, very common in 

 Lough Xeagh, and is present in every month of the year. The maxima occurred 

 in April (15,000,000 filaments in aU catches), but high numbers were present 

 through February, March, AprU, May, and June. The numbers were lower 

 in the hot months of July, August, September, October, and very low from 

 November onwards until January. In the Danish lakes the maxima occur 

 later ; in June and in the ease of other species the maxima synchronize with the 



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